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Low Field NMR Introduction
What is low-field NMR?
NMR instruments can be divided into the following categories according to the strength of the magnetic field:
>3.0T ——-high field strength NMR.
1.0T-3.0T —–Medium field strength NMR.
0.1T-1.0T ——–low field strength NMR.
<0.1T ——Ultra-low field strength NMR.
Low-field NMR mainly refers to NMR instruments with relatively low magnetic field strength. Low-field NMR technology has a very wide range of applications and is still expanding. Low-field NMR technology is mainly based on four aspects for sample analysis and detection:
(1) analysis and detection based on signal amplitude.
(2) based on Analysis and detection of images (two-dimensional distribution of signals).
(3) Analysis and detection based on relaxation time.
(4) Analysis and detection based on diffusion coefficient.
Low-field NMR technology has become more and more widely used in food agriculture, geological exploration, petrochemical, biomedicine, material science and many other fields, and has become an important analysis and testing tool.
The picture below shows a low-field NMR instrument with a magnetic field strength of 0.5T:
The difference between low-field NMR and high-field NMR
The high-field NMR instrument is mainly used to test the chemical structure of molecules and obtain the internal structure information of molecules through chemical shifts.
Low-field NMR is mainly used to test the dynamic information between molecules and obtain molecular motion information and interaction information between molecules through relaxation time; the research domain belongs to the sub-microscopic domain (between molecules), which can measure glass. It is widely used in food industry, petroleum industry, pharmaceutical industry, textile industry, polymer industry, etc..
High-field NMR has high sensitivity, high resolution, and high signal-to-noise ratio, but it requires high sample uniformity, liquid needs to be deionized, solid needs to be powder, and the instrument is expensive, installation requires space, and shielding facilities are required. The instrument requires liquid nitrogen or liquid helium cooling, and the follow-up maintenance cost is very high.
Low-field NMR uses permanent magnets, the equipment is miniaturized, flexible and easy to move, easy to maintain, easy to integrate with other equipment or accessories, and meet the requirements of online high-throughput testing. The low-field NMR instrument has low cost, and the inside of the instrument has been shielded, so the installation site does not need special treatment. Ideal for in-line process inspection, industrial quality control and quality inspection.
The principle of low-field NMR technology
Low-field NMR technology mainly detects H protons but can also be used for fluorine signal testing. After the H-containing sample is excited by a specific frequency of radio frequency, an NMR signal is generated. The H signal corresponds to two main parameters, T1 y T2. By testing the relaxation time of T1 and T2 and modeling, it can be used for food, agricultura, oil exploration, polímeros, contenido de grasa sólida… many aspects of research. A number of methods have been developed to form standard and industry standard methods.
Due to its low equipment cost, low-field NMR has a relatively low threshold for research and use and has a very wide range of applications and is constantly expanding. Because NMR analysis technology has many advantages, such as high speed, alta precisión, multiple parameters can be obtained in one measurement, no sample loss, simple sample preparation, no impact on operator’s health and environment, etc., many other traditional detection methods were originally used. The applications of these methods are being explored using NMR technology.
Low-field NMR instrument architecture
The low-field NMR instrument is divided according to the instrument components, mainly including the industrial computer, the spectrometer system, the radio frequency unit, the gradient unit, the magnet cabinet and the temperature control unit. It consists of five major parts: gradient system, magnet, and constant temperature system. entre ellos, the industrial computer is responsible for receiving the operator’s instructions, and generates various control signals through the sequence generation software to transmit to the various components of the spectrometer system for coordination, and also completes data processing, storage and image reconstruction and display tasks; the radio frequency system is mainly responsible for The radio frequency pulse sequence is transmitted and the sampling signal is received; the gradient system is mainly responsible for generating the gradient magnetic field; the magnet is mainly responsible for providing a uniform and stable main magnetic field; the constant temperature system is mainly responsible for the temperature control in the magnet cabinet.
Application of low-field NMR instruments
1). The application of low-field NMR in the food field:
◆ Test of solid fat content of palm oil, butter and other oils and fats (SFC, Contenido de grasa sólida)
◆ Oil content and moisture content test of oilseeds and seed residues
◆ Solid fat content of chocolate and chocolate-related products
◆ Total fat content
◆ Droplet size analysis of oil-in-water and water-in-oil emulsions
◆ Oil content, water content and protein content of wet and dry food and feed
◆ Food glass transition temperature
◆ Food temperature change research (food heating mode, thermal denaturation, cooking process)
◆ Research on food storage process, moisture migration and moisture distribution
◆ Rapid test of biscuit moisture content
◆ Dry snack oil content test
◆ Research on Gel Hydration
◆ Study on quality change during drying and rehydration process
2). The application of low-field NMR in the agricultural field:
◆ Seed oil moisture content test
◆ Research on the process of seed germination
◆ Automatic breeding of oilseeds (sorting by oil content)
◆ Plant root imaging research
◆ Research on drying of agricultural products
3). The application of low-field NMR in the textile industry:
◆ Oil content in fiber
◆ Polymer coating content
◆ Measurement of fluorinated coatings on fibers and textiles
4). The application of low-field NMR in the material industry:
◆ Xylene soluble content of polypropylene
◆ Polyethylene density and crystallinity
◆ Rubber content of polymers such as acrylonitrile-butadiene-styrene and polystyrene
◆ Elastomer crosslink density
◆ Oil content in sulfur powder samples
◆ Rubber content in polystyrene
◆ Asphalt shingle filler content
◆ Calcium fluoride content in fluorite
◆ Content of plasticizer in polyvinyl chloride (PVC)
◆ Measurement of polyethylene density
◆ Polymer molecular weight test
◆ Zeolite moisture content test
◆ Oil content in ammonium oil
◆ Test of grease content in wastewater
◆ Fluorine content measurement of fluorine-loaded alumina
◆ Research on rubber crosslinking density
◆ Dynamic evaluation of polymer aging and curing process
◆ Research on polymer properties under variable temperature environment
◆ Polymer modification evaluation
◆ Analysis of polymer soft and hard segments
◆ Research on hydrophilicity and hydrophobicity of materials, and research on water absorption process
◆ Study on pore size and pore size distribution of membrane materials
◆ Moisture Migration Analysis of Membrane Materials
◆ Evaluation of hydration of gel materials
◆ Study on wood pore size and pore size distribution
◆ Fluoride content of toothpaste
◆ Melting properties of cosmetics
5). Application of low-field NMR in petrochemical industry:
◆ Hydrogen content of hydrocarbons
◆ Oil content of wax/paraffin
6). The application of low-field NMR in the pharmaceutical industry
◆ Fat and muscle content in live mice and rats
◆ non-contact weighing
◆ Moisture and solvent content of powders and tablets
◆ Contrast agent relaxation time, relaxation rate, in vitro imaging, resonancia magnética in vivo
◆ In vivo rat, mouse MRI, multimodal imaging
7). Application of low-field NMR in suspension system:
◆ Microemulsion relaxation
◆ Adsorption behavior of mixed polymer systems
◆ Competitive adsorption of polymers on silica
◆ Barium titanate precipitation process
◆ Gold and silver nanoparticles accelerate aging effect
◆ Specific surface area of suspended particle system
◆ Evaluate the effect of the grinding process on the surface area of the suspension
◆ Silicon carbide suspension surface area
◆ Mixed particle suspension relaxation behavior
◆ Relaxation behavior of pharmaceutical raw materials during grinding process
◆ Droplet size measurement
◆ Research on powder quality control and dispersion process
8). The application of low-field NMR in the geotechnical energy domain:
◆ Porosity, distribución del tamaño de los poros, permeabilidad, saturation testing
◆ Mechanical damage law and mechanism research
◆ Analysis of soil moisture status, moisture migration, and unfrozen water content of frozen soil
◆ Research on the overall moisture content of sludge and the distribution of moisture content at different levels
◆ Sludge purification treatment process research
◆ Testing of water absorption, water seepage, water holding capacity and water resistance of building materials
◆ Research on cement curing process
9). The application of low-field NMR in the porous material domain:
◆ Pore size distribution research
◆ Research on aperture size
◆ Low temperature nano-pore size test